In principle it is known that thin slab continuous casting mills are especially suitable for producing magnetic steel sheet due to the advantageous control of temperature made possible by inline processing of thin slabs. Thus JP 2002212639 A describes a method for producing grain oriented magnetic steel sheet, wherein a molten metal, which (in wt %) contains 2.5-4.0% Si and 0.02-0.20% Mn as the main inhibitor components, 0.0010-0.0050% C, 0.002-0.010% Al plus amounts of S and Se as well as further optional alloying components, such as Cu, Sn, Sb, P, Cr, Ni, Mo and Cd, the remainder being iron and unavoidable impurities, is formed into thin steel slabs having a thickness of 30-140 mm. In one embodiment of this prior art method, the thin slabs are annealed at a temperature of 1000-1250° C. before hot rolling, in order to obtain optimum magnetic properties in the finished magnetic steel sheet. Furthermore the prior art method requires that the hot strip, which is 1.0-4.5 mm thick after hot rolling, is annealed for 30-600 seconds at temperatures of 950-1150° C., before it is rolled with deformation strains of 50-85% into cold strip. As advantage for using thin slabs as pre-material for producing magnetic steel sheet, it is pointed out in JP 2002212639 A that an even temperature distribution and an equally homogeneous microstructure can be guaranteed over the entire slab cross section due to the small thickness of the thin slabs, so that the strip obtained possesses a correspondingly even characteristic distribution over its thickness.
Another method for producing grain oriented magnetic steel sheet, which however only concerns the production of standard qualities, so-called CGO material (conventional grain oriented material), is known from JP 56-158816 A. In this method a molten metal, which contains (in wt %) 0.02-0.15% Mn as the main inhibitor component, more than 0.08% C, more than 4.5% Si, and in total 0.005-0.1% S and Se, the remainder being iron and unavoidable impurities, is cast into thin slabs having a thickness of 3-80 mm. Hot rolling of these thin slabs begins before their temperature drops below 700 C. In the course of hot rolling the thin slabs are rolled into hot strip having a thickness of 1.5-3.5 mm. The thickness of the hot strip in this case has the disadvantage that the standard final thickness of below 0.35 mm, which is the commercial norm for grain oriented magnetic steel sheet, can only be produced with a cold rolling deformation strain above 76% in a single-stage cold rolling process or by conventional multi-stage cold rolling with intermediate annealing, whereby it is disadvantageous with this method that the high cold deformation strain is not adapted to the relatively weak inhibition through MnS and MnSe. This leads to non-stable and unsatisfactory magnetic properties of the finished product. Alternatively a more elaborate and more expensive multi-stage cold rolling process with intermediate annealing must be accepted.
Further possibilities of producing grain oriented magnetic steel sheet using a thin slab continuous casting mill are extensively documented in DE 197 45 445 C1. In the method developed from DE 197 45 445 C1 and against the background of the prior art known at this time, a silicon steel melt is produced, which is continuously cast into a strand having a thickness of 25-100 mm. The strand is cooled during the solidification process to a temperature higher than 700° C. and divided into thin slabs. The thin slabs are then fed to an equalizing furnace standing inline and heated there to a temperature <=1170° C. The thin slabs, heated in such a manner, are subsequently rolled continuously in a multi-stand hot rolling mill to form hot strip having a thickness of <=3.0 mm, the first forming run being carried out when the rolled strip internal temperature is 1150° C. maximum with the reduction in thickness being at least 20%.
In order to be able to utilize the advantages of the casting/rolling process, as a result of using thin slabs as pre-material, for producing grain oriented magnetic steel sheet, the hot rolling parameters in accordance with the explanations given in DE 197 45 445 C1 must be selected in such a way that the metal always remains sufficiently ductile. In this connection it is stated in DE 197 45 445 C1 that with respect to the pre-material for grain oriented magnetic steel sheet, ductility is greatest if the strand is cooled after solidification to approx. 800° C., then held only relatively briefly at equalizing temperature, for example 1150° C., and is thereby heated homogeneously throughout. Optimum hot rolling ability of such a material is the case therefore if the first forming run takes place at temperatures below 1150° C. with a deformation strain of at least 20% and the strip, starting from an intermediate thickness of 40-8 mm, is brought by means of high pressure inter-stand cooling devices, in two sequential forming runs at most, to rolling temperatures of less than 1000° C. Thus it is avoided that the strip is formed in the temperature range of around 1000° C., which is critical with respect to ductility.
In accordance with DE 197 45 445 C1 the hot strip formed in this way is then cold rolled in one or several stages with intermediate recrystallization annealing to a final thickness ranging between 0.15 and 0.50 mm. The cold strip is finally subjected to recrystallization and decarburization annealing, provided with a predominantly MgO containing annealing separator, then subjected to final annealing in order to form a Goss texture. Finally the strip is coated with an electric insulation and subjected to annealing for relieving stresses.
Despite the extensive proposals for practical use, documented in the prior art, the use of casting mills, wherein typically a strand having a thickness of usually 40-100 mm is cast and then divided into thin slabs, for producing grain oriented magnetic steel sheet remains the exception due to the special requirements, which arise in the production of magnetic steel sheet with respect to molten metal composition and processing control.
Practical investigations demonstrate that pivotal importance is attached to the ladle furnace as regards the use of thin slab continuous casting mills. In this unit the molten steel is fed to the thin slab continuous casting mill and adjusted by heating to the desired temperature for casting. In addition the chemical composition of the steel concerned can be finally adjusted in the ladle furnace by adding alloying elements. Furthermore the slag in the ladle furnace is usually conditioned. When processing steel calmed with aluminium, small amounts of Ca are added to the molten steel in the ladle furnace, in order to guarantee the castability of this steel.
Although in the case of steel calmed with silicon-aluminium, needed for grain oriented magnetic steel sheet, no addition of Ca is required to guarantee castability, the oxygen activity in the ladle slag must be reduced.
The production of grain oriented magnetic steel sheet additionally requires very precise adjustment of the target chemical analysis, that is to say the contents of the individual components must be adjusted very exactly in step with one another, so that depending on the absolute content selected, the limits of some components are very tight. Here treatment in the ladle furnace reaches its limits.
Substantially better conditions can be achieved in this respect by using a vacuum facility. In contrast to ladle degassing however an RH or DH vacuum facility is not suitable for slag conditioning. This is necessary in order to guarantee the castability of melts used for producing grain oriented magnetic steel sheet.